Added slim.before.render default hook

This hook is applied directly before the template render call.

A common use case for this hook would be:
```php
$app->hook('slim.before.render', function () use ($twig) {
  $twig->addGlobal("session",$_SESSION);
});
```
So the latest possible values in the $_SESSION will be available to the view template regardless of where they are set in the app life cycle.